1. Field of the Invention
The field of the present invention is an single overhead cam (SOHC) type internal combustion engine comprising an intake valve driving means interposed between a single cam shaft rotatably disposed above a combustion chamber and a pair of intake valves for converting the rotational motion of the cam shaft into the opening and closing motions of the intake valves, an exhaust valve driving means interposed between the cam shaft and a pair of exhaust valves for converting the rotational motion of the cam shaft into the opening and closing motions of the exhaust valves, and a plug-insertion cylindrical portion for insertion of a spark plug which is to be disposed at a central portion of a ceiling surface of the combustion chamber.
2. Description of the Prior Art
Such SOHC type internal combustion engine is conventionally known, for example, from Japanese Patent Application Laid-open No. 57806/88 and the like.
In the above prior art, a plurality of intake valve-side rocker arms are interposed between a pair of intake valves and a cam shaft, and a pair of exhaust valve-side rocker arms are interposed between a pair of exhaust valves and the cam shaft, so that the pair of intake valves and the pair of exhaust valves are opened and closed by swingably driving the individual rocker arms by cams provided on the cam shaft. In addition, a connection switchover mechanism is provided in the intake valve-side rocker arms and capable of switching-over the connection and disconnection of the rocker arms, in order to improve the output performance of the engine by varying the opening and closing mode of the intake valves in accordance with the operational condition of the engine.
However, the intake valve-side rocker arms are adjacent one another in a location in which the connection switchover mechanism is provided, but the cams for the intake valve-side rocker arms and the cams for the exhaust valve-side rocker arms are provided alternately in an axial direction on the cam shaft and hence, the intake valve-side rocker arms cannot be arranged in a compact manner. This provides an increase in size of the connection switchover mechanism, resulting in an increase in weight of the intake valve-side rocker arm, in a difficulty of improving the dimensional accuracy of the connection switchover mechanism, and in a difficulty of disposing the slide contact portion of the intake valve-side rocker arm with the cam and the operatively connected position of the intake valve-side rocker arm to the intake valve together in a plane perpendicular to the swinging axis of the intake valve-side rocker arm, thereby causing an uneven or eccentric load to act on the intake valve-side rocker arm.